1. Field of the Invention
2. Description of Prior Art
Many porous inorganic supports have been made using a variety of techniques. These include sol-gel synthesis, as taught by U.S. Pat. No. 4,279,779, drying and calcination of slurries of colloidal inorganic particles, with or without the use of binders, e.g., U.S. Pat. Nos. 3,892,580 and 5,128,291, and the use of burnout agents, taught by European Patent 256,359. However, in almost all cases the pores in these supports have been quite small, with diameters typically less than 1 .mu.m (10,000 .ANG.)and most often less than 1000 .ANG., and the preparation methods employed are not easily extended to larger pore diameters. Thus, the pore diameters described in earlier patents have been too small for some catalytic species, e.g., bacteria and other microorganisms. Further, gaseous diffusion is slow in these smaller pores. Many of the above-noted preparation methods also do not lend themselves to the preparation of bodies of arbitrary size and shape without the use of additional processing steps.
As alternatives to inorganic supports, organic materials with large pores have been made, such as macroreticular polystyrene. However, these materials often do not have good dimensional stability: they are subject to swelling in many solvents, and to being deformed under pressure. Further, organic materials often may be degraded under catalytic conditions, e.g., by extreme heat, by oxidizing conditions, or by the action of bacteria or other microorganisms.
U.S. Pat. No. 4,153,510 teaches composites of microbes bonded to the internal surfaces of large-pore inorganic materials. However, it does not teach methods of preparation or compositions which will have the desired properties.
U.S. Pat. No. 4,581,338 teaches the preparation of a spherical, large-pore, catalyst-containing support from diatomite. Large pores are introduced to the support through the addition of burnout agent during processing. Because the diatomite is relatively unreactive, added fluxing agent and high calcination temperatures are required in order to form spheres with good physical strength.
U.S. Pat. No. 3,034,994 teaches the use of extrusion of smaller particles to form macroscopic porous supports. However, if the smaller particles are not strong enough to withstand the rigors of extrusion, then they will be crushed to produce a support with low porosity. Further, if particles with insufficiently large dimensions are used, then no large pores can be formed in the interstices between these particles.
Thus the existing materials and the methods for their preparation have one or more flaws. These include insufficiently large pore diameter in the 0.5 to 100 micron range, difficulty in tailoring pore size to a specified range, the necessity for added burnout agents to introduce porosity, poor physical integrity or dimensional stability, difficulty in processing, and the ability to produce only specified shapes and sizes of bodies. In short, a method of preparation of supports which is easier and more readily tailored to specifications is to be desired. Further benefit would accrue from the ability to add additional reagents/catalysts/adsorbents to the support during processing and the potential for higher cell densities or microbial activities in the finished catalyst, if microbes are used as the catalytically-active species.
3. Objects of the Invention
It is an object of this invention to provide formed inorganic bodies with very large pores and good physical strength, as well as a process for their preparation.
It is a further object of this invention to provide a macroporous catalyst support.
It is a further object of this invention to provide a macroporous catalyst support with specified macropore diameters.
It is a further object of this invention to provide a macroporous catalyst support which allows more rapid diffusion of gases into the support than would be possible in a purely microporous support.
It is a further object of this invention to provide a support with pores large enough to accommodate large catalytic species.
It is a further object of this invention to provide a support for bacteria and other microorganisms.
These and other objects of the current invention will become obvious as the description below proceeds.